This invention relates to methods for gasifying subterranean deposits. More particularly this invention to an improved line-drive method for gasifying such deposits.
Considerable effort has been directed heretofore in the development of processes for conversion of carbonaceous materials such as coal into liquid or gaseous fuels which may be substituted for petroleum-derived fuels. Of particular interest in this connection is the in situ gasification of coal deposits. One well-known method for gasifying such subterranean deposits over a wide area is the so-called line-drive process. In this process well bores are drilled at suitable spacings in substantially parallel rows positioned generally perpendicularly to the direction of the line drive, i.e. the direction in which the burn front is to proceed. In this process, the free oxygen containing gas is injected into a first row of well bores designated injection well bores with ignition and gasification of the coal being accomplished between such first row and an adjacent row of production well bores. When gasification is complete between the first and second rows, the process is continued by converting the production wells to injection wells and utilizing a succeeding row of wells as production wells. In such prior art processes several drawbacks are inherent. In the first place the use of well bores for both injection and production operations results in less efficient gasification of the subterranean deposit than would be accomplished if the wells were used for a single purpose, i.e., injection or production. The cost of completion of dual-purpose wells is particularly high. When linking is attempted from a well which has previously been used for production, the bottom of the well may be burned off, which makes it impossible to control the positioning of the combustion link in the deposit so that combustion can be initiated near the bottom of the subterranean deposit. As is well known, combustion is desirably initiated near the bottom of a carbonaceous deposit for most-effective gasification.
A further operational problem with the prior art line-drive process stems from the fact that combustion takes place in the immediate vicinity and beneath both the injection and production wells. Subsidence is likely to occur at both wells. This may damage the injection wells sufficiently to interfere with the flow of oxygen containing gas and thus interrupt the gasification process; damage to production wells may cause leakage of gasification products from the production wells with consequent environmental hazard.
A still further drawback of conventional short-line-drive configurations is the fact that the active areas of each line-drive pattern are in direct contact with the preceding burned-out pattern. This enhances the likelihood of escape of gasification products to the surface through such burned-out patterns.
A still further problem with the conventional short-line-drive pattern is that it is relatively susceptible to a high rate of water influx because of the considerable length of exposed perimeter in relation to the internal deposit area undergoing conversion.
It is therefore a general object of this invention to provide an improved line-drive pattern for in situ gasification of subterranean carbonaceous deposits.
It is a further and more particular object of this invention to provide a line-drive pattern for in situ gasification of subterranean carbonaceous deposits wherein subsidence is substantially eliminated as an operational problem.
A still further object of this invention is to provide a line-drive pattern for in situ gasification of subterranean carbonaceous deposits wherein large areas may be efficiently produced without environment hazard.
Other objects and advantages of this invention will become apparent from consideration of the detailed description to follow taken in conjunction with the drawing and the appended claims.